Without limiting the scope of the invention, its background is described in connection with growing silicon crystals, as an example.
Heretofore, in this field, silicon crystals have been produced by melting silicon along with a variety of dopants, such as boron or phosphorous, in a quartz crucible located within a furnace. A heater, also located within the furnace, provides heat which liquefies the silicon and the dopants in the crucible at a temperature around 1450.degree. C. Once the silicon is liquefied, a rotating seed is inserted into the melt. The seed is then slowly retracted, allowing the silicon to solidify thereon to create a generally cylindrically shaped silicon crystal. The silicon crystal is subsequently cut into wafers which are cleaned, polished and etched to become an integral part of an integrated circuit.
It has been found, however, that the quality of the integrated circuit is dependent upon the characteristics of the silicon used to manufacture the integrated circuit. For example, the amount of oxygen in the silicon must be strictly controlled, both in terms of the radial oxygen gradient, a ratio of the oxygen near the center of the silicon crystal compared to the oxygen near the outer edge of the silicon crystal, and in terms of the axial oxygen gradient, a ratio of the oxygen near the top of the silicon crystal as compared to the oxygen near the bottom of the silicon crystal.
Oxygen enters the silicon in a reaction along the interface between the liquid silicon and the quartz crucible, forming SiO.sub.x. The oxygen-rich silicon near the surface of the crucible is mixed throughout the liquid silicon due to the swirling effect of silicon flux. Silicon flux is a result of the temperature gradients and density gradients within the silicon melt which are a result of the heat transfer from the heater through the crucible to the silicon melt. The area near the surface of the crucible is hotter than the area near the middle of the crucible, causing the density of the silicon near the surface to be less than the density of the silicon near the middle. This results in a swirling effect in which the silicon near the surface of the crucible rises while the silicon near the center of the crucible falls, thereby mixing oxygen throughout the silicon melt.
To minimize silicon flux, cusp magnets have been utilized. Typically, a pair of cusp magnets are used together, one providing a negatively charged magnetic flux while the other providing a positively charged magnetic flux. The magnets are oriented along the exterior of the furnace in a one above the other configuration. During the production of a silicon crystal, the magnetic flux tends to minimize the silicon flux reducing the mixing of oxygen into the silicon melt and producing a silicon crystal having more uniform characteristics.